Peel the onion and slice the onion and the shallot thinly. Heat some olive oil in a large saucepan and brown the onion and shallots for about five minutes. Peel and crush the garlic and add to the onion.
Crush the walnuts with an electric mixer (powder) and add to the pan as well. Brown for a minute, stirring constantly.
Pour the chestnuts and vegetable broth into the pan. Season with salt and pepper. Bring to a boil, cover and cook for 15 minutes on low heat.
Once the chestnuts are tender, mix with an electric mixer. The soup should be quite thick but add some water if you want.
Before serving add a tablespoon of mascarpone to each place as well as some walnuts and parsley